Optical detection system

ABSTRACT

The present invention pertains to radiant energy systems and more particularly to systems exhibiting the retroreflection principle wherein the system comprises a focusing means and a surface exhibiting some degree of reflectivity positioned near the focal plane of the device, and wherein incident radiation falling within the field-of-view of said system is reflected back in a direction which is parallel to the incident radiation. The present invention has great applicability in military optical system applications for detecting the presence of an enemy employing surveillance equipment and for neutralizing this surveillance capability.

RELATED APPLICATIONS

There is more than one Reissue Application based on U.S. Pat. No.6,603,134. This application is a Divisional of U.S. patent applicationSer. No. 11/197,731, now U.S. Pat. RE40,927, titled “OPTICAL DETECTIONSYSTEM,” filed Aug. 5, 2005 now U.S. Pat. No. Re. 40,927, which is aReissue of U.S. patent application Ser. No. 04/623,186, now U.S. Pat.No. 6,603,134, titled “OPTICAL DETECTION SYSTEM,” filed Mar. 10, 1967,each commonly owned with this application, the entire disclosures ofwhich are here incorporated by reference.

Applicants herein have made the discovery that any type of focusingdevice in combination with a surface, exhibiting any degree ofreflectivity and positioned near the focal plane of the device, acts asa retro-reflector. A retroreflector is defined as a reflector whereinincident rays or radiant energy and reflected rays are parallel for anyangle of incidence within the field-of-view. A characteristic of aretroreflector is that the energy impinging thereon is reflected in avery narrow beam, herein referred to as the retroreflected beam. Thisphenomenon is termed retroreflection.

It is herein to be noted that the term radiant energy includes lightenergy, radio frequency, microwave energy, acoustical energy, X-rayenergy, heat energy and any other types of energy which are part of theenergy spectrum and which are capable of being retroreflected by thedevice, instrument or system sought to be detected.

One type of optical device which exhibits this phenomenon, and thus is aparticular type of retroreflector, is a corner reflector consisting ofthree mutually perpendicular reflecting planes, However, this type ofretroreflector is both difficult and expensive to fabricate.

Due to the applicants discovery, it has now become possible toaccomplish a great many feats heretofore considered impossible, as willbecome more apparent from the discussion to follow hereinafter. In thiscontext it should be noted that the eyes of human beings, as well asthose of animals, operate as retroreflectors. Also, any opticalinstrument which includes a focusing lens and a surface having somedegree of reflectivity, no matter how small, positioned near the focalpoint of the lens, act as a retroreflector, whereby any radiant energyfrom a radiant energy source directed at these instruments is reflectedback towards the source in a substantially collimated narrow beam.

It is therefore the primary object of the present invention to provide amethod and apparatus for detecting objects exhibiting retroreflectioncharacteristics.

It is another object of the present invention to provide a method andapparatus to detect objects having retroreflection characteristics byilluminating the same with a radiant energy source.

It is a more particular object of the present invention to provide amethod and apparatus for scanning an area to detect the presence ofoptical instruments such as binoculars, telescopes, periscopes, rangefinders, cameras, and the like.

It is a further object of the present invention to provide means andapparatus for determining the characteristics of a device exhibitingretroreflection characteristics from a remote location.

It is a further object of the present invention to provide a method andapparatus for detecting optical instruments for rendering theinstruments ineffective and for neutralizing humans utilizing saidinstruments by employing lasers or similar high energy sources.

It is yet another object of the present invention to provide a methodand apparatus for transmitting and receiving radiant energy utilizingconcentric optics.

These and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddiscussion considered in conjunction with the accompanying drawings,wherein:

FIG. 1 is a diagram showing a retroreflection system consisting of alens and a reflector wherein the source radiation is parallel to theoptical axis of the lens.

FIG. 2 is a diagram of a retroreflection system similar to that of FIG.1, wherein the source radiation is not parallel to the optical axis ofthe lens.

FIG. 3 is a diagram of a retroreflection system similar to FIG. 1wherein the lens is imperfect so as to form an image rather thanfocusing at a single point.

FIG. 4 is a diagram of a retroreflection system wherein the reflector isobliquely positioned with respect to the optical axis of the lens.

FIG. 5 is a diagram of a human eye, wherein there is depicted theretroreflection characteristics thereof.

FIG. 6 is a schematic representation depicting a beam splitting opticalsystem for transmitting and receiving radiant energy.

FIG. 7 is a schematic representation depicting a concentric opticalsystem for transmitting and receiving radiant energy.

FIG. 7a is a schematic representation of another embodiment of theconcentric optical system depicted in FIG. 7.

FIG. 7b is a schematic representation of still another embodiment of theconcentric optical system depicted in FIG. 7.

FIG. 8 is a schematic representation depicting an ordinary telescope asan image forming system having retroreflection characteristics.

FIG. 9 is a schematic representation depicting one half of an ordinarybinocular as an image forming system having retroreflection.

FIG. 10 is a schematic representation depicting an ordinary periscope asan image system having retroreflection characteristics.

FIG. 11 is a schematic representation depicting an ordinary camera as animage forming system having retroreflection characteristics.

FIG. 12 depicts a system for scanning an area to detect the presence ofoptical instruments by utilizing the retroreflection characteristicsthereof and for neutralizing observers using said optical instruments,and/or rendering the instruments ineffective.

FIG. 13 is a diagram of a radar system, and more particularly of a radarantenna which is to be detected in accordance with the principles of thepresent invention.

FIG. 14 depicts the waveforms obtained during the detection of the radarsystem shown in FIG. 13.

In accordance with the general principles of the present invention anoptical system consisting of a focusing lens and a reflective surfacepositioned near the focal plane of said lens has radiant energy rayssupplied thereto by a radiant energy transmitter. The radiant energyrays reflected by the optical system due to its retroreflectioncharacteristics are recovered by a radiant energy receiver to therebydetect the presence and relative position of said optical system. Theoutput of the radiant energy receiver may be applied to a utilizationmeans for determining the characteristics of the retroreflector or fordirecting a weapon means.

Referring now to the drawings and more particularly to FIG. 1 thereof,there is shown an optical system consisting of a lens 20 and areflective surface 22, which herein is a mirror, positioned in the focalplane 24 of the lens 20. Rays of radiation 26 and 28, respectively, aredirected towards the system, and more particularly towards the lens 20,from a radiation source (not shown); the incident rays in the presentillustration being parallel to the optical axis 30 of the lens. It isherein to be noted that for the purpose of clarity the incident rays areherein shown as being confined to the top half of the lens 20. Theincident rays 26 and 28 are refracted by the lens 20 and focused at thefocal point 32 of the lens, which focal point lies on the mirror 22. Therays are then reflected by the mirror so that the angle of reflectionequals the angle of incidence, and are returned to the lower half of thelens where they are again refracted and emerge therefrom asretroreflected rays 26R and 28R. The rays 26R and 28R are returned tothe radiation source parallel to the incident rays 26 and 28 thereof.However, as shown in the drawing, the relative positions of the rays 26and 28 are inverted so that the image returned to the radiation sourceis also inverted.

In the optical system depicted in FIG. 2, similar parts are de-noted bysimilar reference numerals. In this system the rays 34 and 36 are notparallel to the optical axis 30A of both the lens 20A and the mirror22A, the mirror 22A being positioned in the focal plane 24A of the lens.The rays 34 and 36 are refracted by the lens 20A and focused at a point37 removed from the optical axis but still on the focal plane. The rays34 and 36 are reflected by the mirror. Both of the rays 34 and 36 wouldnormally emerge from the lens as retroreflected rays 34R and 36R, afterrefraction by the lens, and would be returned to the source of the rays34 and 36 in a direction parallel thereto. However, since the lens 20Ais of finite size, the reflected ray 34R will miss the lens and will notbe retroreflected. The loss of reflected rays in this manner is called“vignetting”.

In the system depicted in FIG. 3 wherein similar parts are de-noted bysimilar reference numerals, the lens 20B is assumed to be imperfect;i.e., it has aberrations. In this case the rays 38 and 40 are parallelto the optical axis 30B but are not focused at a single point on thefocal plane 24B, and instead form an image on the mirror 22B, whichimage is referred to as the circle of confusion. In most practicaloptical systems there are circles of confusion and the mirror isnormally positioned at the plane of least circle of confusion, hereindepicted by the reference numeral 42. Thus, the image formed on themirror by means of the rays 38 and 40 can be considered to be a radiantsource, and the retroreflected rays 38R and 40R exit from the lens 20Bsubstantially parallel to each other. This is possible since eachemerging ray can be paired with a parallel incident ray which radiatesfrom a common point of the image source located at the mirror 22B.

In the system depicted in FIG. 4, the reflecting surface or mirror 22C,and its axis 44, is tilted with respect to the optical axis 30C of lens20C. However, the ray 48 is again retroreflected by the system and theretroreflected ray 48R is returned parallel to the incident ray 48. Theretroreflected ray 46R, due to the ray 46, is lost because ofvignetting.

The concept set forth herein above in conjunction with FIG. 3, that theretroreflected rays be considered as radiating from a source on theimage plane, is highly significant. With this concept in mind, it willbe readily apparent that even if the retroreflecting surface isdispersive, curved, or tilted, (as shown in FIG. 4), the system willstill exhibit retroreflective properties for any and all rays which arereturned to the lens by the reflecting surface.

The rays retroreflected by the optical systems depicted in FIGS. 1 to 4are in the form of a narrow, substantially collimated beam having a highradiant flux density. It is to be noted that there is an actual increasein the radiant flux density of the retroreflected beam due to thenarrowing thereof. This increase in radiant flux density is hereintermed optical gain.

For example, if the irradiance produced by the radiating source at thecollecting lens in FIG. 3 is 100 watts/cm² and the area of the lens is100 cm², then the radiant flux at the image or focal plane (circle ofconfusion) is

${\frac{100\mspace{14mu}{watts}}{{cm}^{2}} \times 100\mspace{14mu}{cm}^{2}},{{or}\mspace{14mu} 10^{4}\mspace{14mu}{{watts}.}}$

It is a characteristic of a retroreflector to return the retroreflectedenergy or rays in a very narrow beam. The dimensions of theretroreflected beam is a function of the angular resolution of theretroreflector which includes the lens and the reflecting surface.

The solid angle into which the incident radiant flux will beretroreflected is determined by the square of the angular resolution ofthe retroreflector. If, for example, the resoltuion of the opticalsystem is 10⁻⁴ radians, the solid angle into which the retroreflectedbeam will be returned is 10⁻⁸ steradians. One steradian being the solidangle subtended at the center of a sphere by a portion of the surface ofarea equal to the square of the radius of the sphere. Thus at a distanceof 10⁴ cm from the focal plane the area of the retroreflected beam isonly 1.0 cm². The retroreflector, by radiating into such a small solidangle, has radiant intensity of

$\frac{10^{4}\mspace{14mu}{watts}}{10^{- 8}\mspace{14mu}{steradian}},{{or}\mspace{14mu} 10^{12}\mspace{14mu}{watts}\text{/}{{steradian}.}}$

In order to obtain a measure of the optical gain we must compare theretroreflector to a standard or reference. This reference has been takento be a diffuse surface known in the art as a Lambertian radiator. Ifthe 10⁴ watts of incident radiant flux were simply re-radiated in aLambertian manner; i. e., into a solid angle of 3.14 (π) steradians, theradiant intensity would be

$\frac{10^{4}\mspace{14mu}{watts}}{3.14\mspace{14mu}{steradians}},{{or}\mspace{14mu} 3.1 \times 10^{3}\mspace{14mu}{watts}\text{/}{{steradian}.}}$Thus, the retroreflector has an overall optical gain equal to

$\frac{10^{12}\mspace{14mu}{watts}\text{/}{steradian}}{3.1 \times 10^{3}\mspace{14mu}{watts}\text{/}{steradian}},{{or}\mspace{14mu} 3.14 \times 10^{8}}$

Although there is no actual increase in radiant flux, the retroreflectorhas a radiant intensity which is 3.14×10⁸ greater than that of aLambertain radiator which emits the same radiant flux. Thus, forexample, a telescope having a collecting area of 100 cm² and an angularresolution of 0.1 milliradian would appear similar in size to about3.5×10⁸ cm² of a diffuse or Lambertian radiator.

It should be noted that in almost all cases, the retroreflector will bedisposed within an environment that produces background radiation in aLambertian manner. Thus, the radiant intensity of the retroreflector isso much greater than that of a Lambertian radiator that it is easilydiscernible from the background, even when, (as shown in FIG. 2) a largepercentage of the retroreflected radiant flux is lost due to vignetting.

It is herein to be noted that the radiant intensity of theretroreflected beam is dependent upon the characteristics of the opticalsystem employed. If an optical system of the type shown in FIGS. 1, 2,and 4 were possible and there were no loss of energy (power) enteringthe system, then the radiant intensity gain would be almost infinitesince the energy would be retroreflected in an almost perfectlycollimated beam, i.e. a retroreflected beam whose divergence angle isalmost zero. However, almost all optical systems resemble that shown inFIG. 3 and the factor which determined the divergence angle of theretroreflected beam is the size of the circle of confusion and moreparticularly, the least circle of confusion. The size of the leastcircle of confusion is dependent upon the resolution of the system andin particular upon the resolution of the focusing lens. Thus, the lessaberrations in the lens, the better the resolution, the smaller thecircle of least confusion, the smaller the divergence angle of theretroreflected beam, and thus the greater the optical gain.

Referring to FIG. 5, there is shown a magnified cross-sectional view ofa human eye denoted generally by the reference numeral 50. The eyeincludes a cornea 52, an anterior chamber 54, a lens 56, and a retina58. The retina has a small portion or point 60 thereon termed the yellowspot or macula lutea, which is approximately 2 mm in diameter. At thecenter of the macula lutea is the fovea centralis 62 whose diameter isapproximately 0.25 m. The acuity of vision is greatest at the maculalutea and more particularly at the fovea centralis. Thus, the eye isalways rotated so that the image being examined or the rays enteringthereon fall on the fovea 62. As seen in FIG. 5, rays 64 and 66 enterthe eye and pass through the cornea 52 and the anterior chamber 54 andare refracted by the lens 56 and focused on the fovea centralis portion62 of the retina 58. The rays are then reflected, passing through thelens 56, anterior chamber 54 and cornea 52 and emerge therefrom asretroreflected rays 64R and 66R which are parallel to the rays 64 and66. Thus, it is seen that even the human eye acts as a retroreflector.

Referring now to FIG. 6, there is shown an optical system fortransmitting and receiving radiant energy, the more particularly a beamsplitter for transmitting radiant energy and for receiving or recoveringa portion of said radiant energy.

The beam splitter includes an optical bench 70 having an optical systemconsisting of a lens 72 and a rotating pattern or reticle 74, which mayalso be a modulator, said system being placed on said bench. The beamsplitter also includes a radiant energy source 76, a collimator 78, athin plate of glass 80 having a semi-reflective coating thereon, adetector 82. In the operation of the beam splitter, the radiant energyfrom the source 76 is collimated to form a beam by the collimator 78 andthe beam is directed upon the glass plate 80, a portion of the energy inthe beam being reflected and a portion of the energy in the beam beingtransmitted by the glass plate. The energy is then transmitted down theoptical bench 70 where the lens refracts the transmitted energy andfocuses the beam upon the reticle 74 from whence is is retroreflectedback to the glass plate. A portion of the retroreflected energy passesthrough the glass plate and is lost, and a portion thereof is reflectedby the glass plate and detected by means of the detector and the outputthereof is then fed to the utilization means 83. The detector 82 is thuseffectively positioned within or concentric with the retroreflectedenergy beam without affecting the transmission of radiant energy fromthe source to the optical system. The energy obtained by the utilizationmeans can be used to obtain the spectral and temporal characteristics ofthe retroreflected beam, and may the be compared with the transmittedbeam to determine various characteristics of the optical system beinginvestigated. It will be apparent that the use of this test instrumentmakes possible the investigation and characterization of optical systemsin terms of recording the retroreflective characteristics thereof.

The rotating pattern or reticle 74 can be replaced with a reflectivesurface and a modulator placed on the light incident side of the lens72. The modulator can then be tilted so that none of the light reflectedfrom its surface returns to the beam splitter 80 to be reflected to thedetector 82. The only light then returning to the detector 82 will bethat modulated by the modulator and reflected back from the reflectivesurface replacing the reticle 74.

FIG. 7 depicts a folded concentric optical system for transmitting andreceiving radiant energy—also known as an optical transceiver. Theoptical transceiver 84 includes a primary mirror 86 having asubstantially parabolic shape, a secondary mirror 88 having a planarconfiguration, a radiant energy source 90, a detector 92 and autilization means 94. The primary mirror has an aperature 96 concentricwith its principal axis and the principal axis of the secondary mirroris aligned so as to be coaxial therewith. The light source and detectorare also aligned with the mirrors so that all of the aforesaid elementsare concentrically disposed with respect to each other. The light sourceis positioned adjacent to the nonreflecting surface of the primarymirror while the detector is positioned adjacent to the nonreflectingsurface of the secondary mirror.

In the operation of the transceiver 84, rays 98 and 100 are emitted bythe radiant energy source 90, and impinge upon the secondary mirror 88,from whence they are reflected and impinge upon the primary mirror 86.The rays are then reflected by the primary mirror and directed towardsan optical instrument 102 which exhibits retroreflectivecharacteristics. The incident rays are retroreflected by the opticalinstrument 102 and are returned as retroreflected rays 98R and 100R. Therays 98R and 100R return in a direction parallel to the rays 98 and 100and impinge upon the primary mirror 86 and are reflected thereby towardsthe detector 92 where they are detected, and the detector output signalis then fed to the utilization means 94.

As discussed previously, the term optical instruments exhibitingretroreflective characteristics include the eyes of animals and humans.

A second embodiment of a folded concentric optical transceiver is shownin FIG. 7a, wherein similar parts are denoted by similar referencenumerals.

In this embodiment the light source 90A is positioned adjacent to thenonreflecting surface of the secondary mirror 88A and the detector 92Ais positioned adjacent to the nonreflecting surface of the primarymirror 86A.

In the operation of the transceiver 84A, rays 104 and 106 are emitted bythe radiant energy source 90A, and impinge upon the primary mirror 86A,from whence they are reflected towards the optical instrument 102A. Therays are retroreflected by the optical instrument and are returned asretroreflected rays 104R and 106R. The rays 104R and 106R return in adirection parallel to the rays 104 and 106 and impinge upon the primarymirror and are reflected thereby towards the secondary mirror throughthe aperture 96A to the detector 92A, and the output signal of thedetector is then fed to the utilization means 94A.

A third embodiment of a folded concentric optical transceiver isdepicted in FIG. 7b, wherein similar parts are denoted by similarreference numerals.

In this embodiment, the detector 92B is once more positioned adjacent tothe nonreflecting surface of the secondary mirror 88B and the radiantenergy source 90B is positioned between the reflecting surfaces of theprimary mirror 86B and the secondary mirror 88B. There is also includeda collector 108, which may be an elliptically shaped mirror forcollecting the spurious radiation rays from the source 90B andreflecting back upon the source, wherefrom they are directed upon thesecondary mirror and ultimatel directed toward the optical instrument102B.

In the operation of the transceiver 84B, energy from the radiant energysource 90B impinges upon the secondary mirror 88B, and more particularlyrays 110 and 112 so impinge. These rays are reflected by the secondarymirror towards the primary mirror, from where they are once morereflected towards the optical instrument 102B. The incident rays 110 and112 are then retroreflected by the optical instrument and returned asretroreflected rays 110R and 112R. The rays 110R and 112R return in adirection parallel to the rays 110 and 112 and impinge upon the primarymirror and are reflected thereby towards the detector 92B where they aredetected and the output thereof is then fed to the utilization means94B.

It is herein to be noted that although the folded optical transceiversdepicted in FIGS. 7, 7a, and 7b have been shown as being concentric, itis also possible to employ the above type of transceivers wherein theiroptical characteristics are not concentric. However, it has been foundfrom the view-point of efficiency and efficacy that the concentricoptical transceivers are more desireable.

FIG. 8 is an optical schematic representation of a telescope having anobjective lens 116, a reticle 118, a pair of erector lenses 120 and 122,a field lens 124, and an eyelens 126.

Thus, when rays 128 and 129 are directed towards the objective 20 lens116, they are focused on the reticle 118 and retroreflected thereby toproduce retroreflected rays 128R and 129R respectively, whose directionis opposite and parallel to that of the incident rays 128 and 129. Thus,the combination of the objective lens 116, and the reticle 118 form aretroreflective optical instrument, in and of themselves.

It is herein to be noted that even if the reticle 118 is merely plainglass, as in most cases it is, it still exhibits some degree ofreflectivity, which reflectivity gives rise to the retroreflected rays128R and 129R.

It is herein also to be noted that incident rays passing through thetelescope to the eye of the observer are also retroreflected by the eyeof the observer. Thus, there is in effect, two retroreflective opticalsystems and thus two retroreflective signals.

FIG. 9 is an optical schematic representation of one half of a binocularand comprises an objective lens 132, a first porro prism 134, a secondporro prism 136, a reticle 138, a field lens 140, and an eyelens 142.When a ray such as 144 is incident on the objective lens 132, it isfocused thereby on the reticle 138, after passing through the porroprisms 134 and 136. It is herein to be noted that although the ray 144is directed along a path which is not straight; i.e., there are severalright angle bends therein, the entire path is still part of the focalpath of the instrument. Thus, the ray 144 is focused on the reticle 138,causing the same to be retroreflected as ray 144R which then goesthrough a path similar to that of ray 144 and emerges from the objectivelens 132 in a direction which is opposite and parallel to that of theincident ray 144. It is to be noted that the description herein abovedescribing a single ray is for purposes of simplicity of explanation.

FIG. 10 is an optical schematic representation of a periscope. Theperiscope includes a window 146, an objective prism 148, an objectivelens 149, an amici prism 150, an erecting prism assembly 152, a reticle154, a field lens 156, an eyelens 158, and a filter 160. An incident ray162 enters the periscope through the window 146, then passes through theprism 148, objective lens 149, amici prism 150, and erecting prismassembly 152 to the reticle 154 whereon the incident ray is reflectedand emerges from the periscope as retroreflected ray 162R whosedirection is opposite and parallel to the incident ray 162. Again it isto be noted that the description above describing a single ray is merelyfor the purpose of simplicity of explanation.

FIG. 11 is an optical schematic representation of a camera. The cameraincludes a lens 164, a shutter 166, and film 168. In the operation ofthe camera when a picture is taken the shutter opens and incident rays170 and 171 are focused on the film 168 through an aperture 172 in theshutter, by means of the lens 164. These rays are then reflected by thefilm and emerge from the lens as retroreflected rays 170R and 171R.

It is to be noted that most, if not all, optical systems will have areflecting surface such as a reticle, a lens, or a prism in the focalplane, and the incident radiation will be retroreflected by any suchsurface.

Referring now to FIG. 12, there is shown one embodiment of a system fordetecting the presence of an optical instrument, for tracking saidinstrument, and for neutralizing observers utilizing said instrumentand/or rendering the instrument ineffective.

The system includes a scanner 180, including an optical searching means182, such as a source of infrared light, a detector 184, and a laser186. It is herein to be noted that the search means 182 and the detector184 may be combined in the form of a transceiver as describedhereinbefore in conjunction with FIGS. 7, 7a, and 7b. The scanner 182 iscontrolled by a scanning and positioning means 188, which includes aservo motor (not shown.) The scanning and positioning means 188 ispowered by a power and control means 190, which means also suppliespower for the scanner 180, and a utilization system 192.

In the operation of the system, the scanner 180 is caused to scan apreselected area by means of the scanning and positioning means 188, themeans 188 being programmed by the utilization system 192. The opticalsearching means emits rays 194 and 195, when these rays impinge upon anoptical instrument 196 exhibiting retroreflective characteristics, ashereinbefore described, they are retroreflected as retroreflected rays194R and 195R respectively, and detected by the detector 184 and thedetector output is then fed to the utilization system 192. Theutilization system may be programmed to merely track the instrument 196,in which case, this information would be fed to the scanning andpositioning means 188 and thence to the scanner 180 causing it to tracksaid instrument. However, if it is desired to neutralize the observerusing the instrument, or to render the instrument ineffective, then theutilization system 192 will feed a signal to the laser 186 activatingthe same and causing a high intensity laser beam to be directed at theinstrument, thereby accomplishing the aforementioned objects.

It is herein to be noted that although the present system has beendescribed as employing a laser, it is also possible to use any otherhigh energy system, weapon, or weapon system.

With the present system, it will be readily apparent to those skilled inthe art, that a hostile satellite orbiting the earth and employingoptical surveillance equipment to monitor a country's activities can bedetected and its surveillance capability destroyed.

It is herein again to be noted that the aberrations in almost alloptical instruments cause a small divergence of the retroreflected rays,the amount of said divergence being governed by the resolution of theretroreflector. As a practical matter the angular resolution of opticalsystems such as binoculars, periscopes, telescopes, cameras, and opticalsystems carried by missiles will be between about 10⁻³ and 10⁻⁵ radianswhich produce retroreflected beams of 10⁻⁶ to 10⁻¹⁰ steradians. At arange of 1,000 feet the area of these beams would be 1.0 and 10⁻⁴ ft²respectively. This divergence is so small so that the retroreflectedrays are substantially collimated.

It is herein to be noted that in microwave application corner reflectorshave been utilized for retroreflecting purposes. However, the presentinvention enables the detection of microwave apparatus, such as antennasand the like which do not have a corner reflector as an integral partthereof, by utilizing the inherent retroreflection characteristics ofthe apparatus as hereinbefore discussed. Thus, this apparatus andsystems exhibiting the retroreflection phenomenon can be similarlydetected by the use of radio frequency, microwave, X-ray, acoustical orany similar types of energy directed thereat.

In many microwave antenna systems microwave lenses are utilized in placeof reflectors for the purposes of obtaining wide angle scanning ascompared with the system bandwidth. These microwave lenses exhibitcharacteristics which are equivalent to the optical lenses hereinbeforediscussed, and thus a detailed explanation of the retroreflection ofmicrowave and similar types of energy by these lenses, in conjunctionwith a reflective surface, will be readily apparent to those skilled inthe art.

In this connection, FIG. 13 is an illustration of a radar system whichis to be detected by means of the retroreflection principles of thepresent invention. The radar system is generally indicated by thereference numeral 200 and includes a parabolic disk antenna 202 having afeed 204 whose impedance mismatch is lowest at the operating frequencyof the radar system 200.

When the radar system 200 is in an off condition, the resonant frequencyof the antenna feed 206 can be detected by beaming swept frequencymicrowave energy at the system such as by utilizing a variable frequencyklystron (not shown) or the like.

The pulses produced by the klystron are indicated as 210 in thewaveforms shown in FIG. 14. The wave energy 210 is retroreflected by theparabolic disk antenna 202 because the parabola focuses the energy atthe feed horn which in turn is mismatched. Hence, the energy reflectedfrom it is recollimated by the parabola similar to the optical systemdescribed heretofore. The energy is detected in a suitable manner andproduces the waveforms indicated at 212 in FIG. 14, until such time thatthe frequency of the klystron is equal to the operating frequency of thefeed 206. When this occurs, the energy beamed to the radar system isfocused on the feed horn, absorbed by the feed 206 and is therefore notretroreflected. This results in the waveform indicated as 214 in FIG. 1514. The dip or drop in power level indicates absorption of the beamedenergy and thus the frequency of the operation of the radar system isnow known. By further analysis of the retroreflected waves it ispossible to obtain even more information concerning the electrical andmechanical characteristics of the radar system 200, such as the type ofantenna system being utilized, its scan angle, its beamwidth, its gain,etc.

It will be apparent to those skilled in the art that if the antenna werea sonar disk and acoustical energy were directed threat, the acousticalenergy would be retroreflected and the retroreflected acoustical energywould be capable of detection.

It is thus again reiterated that although only a few types of radiantenergy have herein been discussed, any type of energy which can beretroreflected may be employed.

While we have shown and described various embodiments of our invention,there are many modifications, changes, and alterations which may be madetherein by a person skilled in the art without departing from the spiritand scope thereof as defined in the appended claims.

1. The method of detecting an uncooperative optical system including afocusing means and a surface exhibiting some degree of reflectivitydisposed substantially in the focal plane of said focusing means, saidmethod comprising the step of directing optical energy at said opticalsystem whereby that portion of said energy incident upon said opticalsystem is retroreflected with an optical gain to thereby form a beam ofretroreflected optical energy, and the step of detecting saidretroreflected optical energy having a radiant flux density in excess ofa preselected value to thereby indicate the presence of said opticalsystem.
 2. The method of claim 1, including the step of scanning apredetermined geographical area to detect the presence of an opticalsystem therein.
 3. The method of claim 2, including the step of trackingsaid optical system after the presence thereof has been detected.
 4. Themethod of claim 3, including the step of directing a weapon at theposition of said optical system after the detection thereof.
 5. Themethod of claim 1, wherein the radiant energy directed at said opticalsystem is in the nonvisible region.
 6. The method of claim 1, whereinthe radiant energy directed at said optical system is light energy inthe nonvisible region.
 7. The method of claim 6, wherein the lightenergy in the nonvisible region is infrared.
 8. The method of claim 4,wherein said weapon is a laser.
 9. The method of claim 1, wherein theradiant energy is in the ultraviolet portion of the electromagneticspectrum.
 10. The method of claim 1, wherein the radiant energy is X-rayenergy.
 11. The method of claim 1, wherein the radiant energy compriseshigh energy particles related to quantum mechanics.
 12. The method ofclaim 1, wherein the radiant energy is acoustical energy.
 13. The methodrecited in claim 1 wherein said optical system is a telescope.
 14. Themethod recited in claim 1 wherein said optical system is a binocular.15. The method recited in claim 1 wherein said optical system is aperiscope.
 16. The method recited in claim 1 wherein said optical systemis a human eye.
 17. Apparatus for detecting the presence of anuncooperative optical system including a focusing means and a surfaceexhibiting some degree of reflectivity disposed substantially in thefocal plane of said focusing means, said apparatus comprising means forproducing radiant energy, means for directing said energy toward saidoptical system whereby said energy is retroreflected with an optical bysaid optical system, and means for detecting said retroreflected energyhaving a radiant flux density in excess of a preselected value tothereby indicate the presence of said optical system.
 18. Apparatus inaccordance with claim 17 wherein said means for producing radiant energyis a radiant energy source operative in the nonvisible region. 19.Apparatus in accordance with claim 17, wherein said means for producingradiant energy is a radiant energy light source.
 20. Apparatus inaccordance with claim 19, wherein said radiant energy light source is aninfrared source.
 21. Apparatus in accordance with claim 17, wherein saidmeans for producing radiant energy, said means for directing said energytoward said optical system, and said means for detecting the energyretroreflected by said optical system, form an optical transceiver. 22.Apparatus in accordance with claim 21, wherein said means for producingrays of radiant energy, said means for directing said rays toward saidoptical instrument, and said means for detecting the rays retroreflectedby said optical instrument are concentrically disposed with respect toone another.
 23. Apparatus in accordance with claim 22, wherein saidmeans for producing radiant energy, said means for directing said energytoward said optical system, and said means for detecting said energyretroreflected by said optical system are concentrically disposed withrespect to one another.
 24. Apparatus in accordance with claim 22,wherein said means for producing radiant energy comprises a radiantenergy source said means for directing said energy toward said opticalsystem comprises a primary mirror having a substantially parabolicconfiguration, and said means for detecting said retroreflected energycomprising a detector said primary mirror, and a secondary mirror havinga substantially planar configuration said primary mirror having anaperture concentric with the principal axis thereof, said radiant energysource being positioned adjacent the non-reflecting surface of saidsecondary mirror, in the focal plane of said primary mirror, saidsecondary mirror being positioned adjacent said primary mirror, andhaving the reflecting surface of said secondary mirror facing thereflecting surface of said primary mirror, and said detector beingpositioned adjacent the non-reflecting surface of said primary mirror,being in axial alignment with the aperture thereof, being positioned inthe focal plane of said detection means.
 25. Apparatus in accordancewith claim 22, wherein said means for producing radiant energy comprisesa radiant energy source, said means for directing said energy towardsaid optical system comprises a collecting mirror having a substantiallyelliptical configuration a primary mirror having a substantiallyparabolic configuration, and a secondary mirror having a substantiallyplanar configuration, said means for detecting said retorreflectedenergy comprising a detector, and said primary mirror, said primarymirror having an aperture concentric with the principal axis thereof,said secondary mirror being positioned with the reflecting surfacethereof facing the reflecting surface of said primary mirror, saidradiant energy source being positioned between the reflecting surfacesof said primary and secondary mirrors, and in axial alignment with saidmirrors, said collecting mirror being positioned adjacent thenon-reflecting surface of said primary mirror, in axial alignment withthe aperture thereof, and said detector being positioned in the focalplane of said direction means adjacent the non-reflecting surface ofsaid secondary mirror in the focal plane of said primary mirror. 26.Apparatus in accordance with claim 21, wherein said means for producingincident radiant energy is a radiant energy light source operative inthe nonvisible region.
 27. Apparatus in accordance with claim 23,wherein said radiant energy light source is an infrared source. 28.Apparatus in accordance with claim 17, wherein said means for directingsaid incident energy towards said optical system having scanning meansoperatively associated therewith to cause said rays to scan apredetermined geographical area to detect and locate said opticalsystem.
 29. Apparatus in accordance with claim 28, including trackingmeans operatively associated with said scanning means to thereby trackthe movement of said optical system after detection thereof. 30.Apparatus in accordance with claim 28, including weapon meansoperatively associated with said tracking means for use against saidoptical system after detection thereof.
 31. Apparatus in accordance withclaim 30, wherein said weapon means is high energy source.
 32. Apparatusin accordance with claim 31, wherein said high energy source is a laser.33. The apparatus recited in claim 17 wherein said optical system is atelescope.
 34. The apparatus recited in claim 17 wherein said opticalsystem is a binocular.
 35. The apparatus recited in claim 17 whereinsaid optical system is a periscope.
 36. The apparatus recited in claim17 wherein said optical system is a human eye.
 37. Apparatus formeasuring the retroreflective characteristics of an optical systemconsisting of at least a focusing means and a surface exhibiting somedegree of reflectivity disposed substantially in the focal plane of saidfocusing means, said apparatus comprising a radiant energy source,detection means, measuring means connected to said detection means, andmeans for directing said radiant energy produced by said source at saidoptical system, whereby said radiant energy is retroreflected with anoptical gain by said optical system and detected by said detecting meansand the output thereof is coupled to said measuring means.
 38. Anoptical system accordance with claim 37, including means disposedbetween said radiant energy source and said optical system fortransmitting a portion of the radiant energy produced by said radiantenergy source toward said optical system, and for transmitting a portionof said energy retroreflected by said optical system toward saiddetecting means.
 39. An optical system in accordance with claim 38,wherein said directing means and said detecting means are substantiallyconcentric.
 40. The method of detecting the presence of devices whichexhibit the phenomenon of retroreflection, said method comprising thestep of directing radiant energy at said devices whereby said radiantenergy is retroreflected with an optical gain by said devices, and thestep of detecting said retroreflected radiant energy which is in excessof a preselected radiant flux density level to thereby indicate thepresence of said devices.
 41. The method of claim 40, including the stepof analyzing said retroreflected radiant energy to thereby determine thespatial and temporal characteristics of said devices.
 42. Apparatus fordetecting the presence of devices which exhibit the phenomenon ofretroreflection, said apparatus comprising means for producing radiantenergy, means for directing said energy toward said devices whereby saidenergy is retroreflected with an optical gain by said devices, and meansfor detecting said retroreflected energy which is in excess of apreselected radiant flux density level to thereby indicate the presenceof said devices.
 43. apparatus for measuring the retroreflectivecharacteristics of devices which exhibit the phenomenon ofretroreflection, said apparatus comprising means for producing radiantenergy, means for directing said energy toward said devices whereby saidenergy is retroreflected with an optical gain by said devices, means fordetecting said retroreflected energy which is in excess of a preselectedradiant flux density level to thereby indicate the presence of saiddevices, and means for analyzing said detected energy to therebydetermine the characteristics of said devices.
 44. The method ofdetecting an uncooperative and nonradiating microwave antenna systemconsisting of at least a microwave focusing means and a microwave feedhorn disposed substantially at the focal point of said focusing means,said method comprising the step of directing swept frequency microwaveenergy at said antenna system whereby substantially all energy at theoperating frequency of said antenna system which is impingent thereon isfocused by said focusing means and absorbed by said feed horn and energyof any other frequency is retroreflected by said antenna system with anenergy density gain to thereby form a beam of retroreflected microwaveenergy, and the step of detecting said retroreflected energy having anenergy density in excess of a preselected value to thereby indicate thepresence of said antenna system.
 45. The method recited in claim 44further including the step of determining the frequency at which theenergy density of said retroreflected energy is of a minimum level tothereby determine the operating frequency of said antenna system. 46.The method recited in claim 44 further including the step of analyzingany temporal characteristics of said energy retroreflected by saidantenna system.
 47. The method recited in claim 44 further including thestep of analyzing any spatial characteristics of said beam of energyretroreflected by said antenna system.
 48. A method of detectingcharacteristics of an object within an optical system, comprising:transmitting energy at an object included in an optical system havingretroreflective characteristics, wherein the optical system includes alens and the object includes a surface exhibiting some degree ofreflectivity disposed substantially in a focal plane of the lens;receiving reflected radiant energy with an optical gain afterretroreflection of the radiant energy; and detecting the reflectedradiant energy after retroreflection to determine at least onecharacteristic of the object.
 49. The method of claim 48, wherein the atleast one characteristic includes any optically detectable property ofthe object.
 50. The method of claim 48, wherein the at least onecharacteristic includes a relative position of the object within theoptical system.
 51. An apparatus for detecting characteristics of anobject within an optical system, the apparatus comprising: a radiantenergy source for transmitting energy at an object included in anoptical system having retroreflective characteristics, wherein theoptical system includes a lens and the object includes a surfaceexhibiting some degree of reflectivity disposed substantially in a focalplane of the lens; and a detector for detecting received reflectedradiant energy with an optical gain after retroreflection of the radiantenergy to determine at least one characteristic of the object.
 52. Theapparatus of claim 51, wherein the at least one characteristic includesany optically detectable property of the object.
 53. The apparatus ofclaim 51, wherein the at least one characteristic includes a relativeposition of the object within the optical system.